The sintering of precious metal catalysts is an important problem for the automotive and chemical process industry. As these precious metal catalysts sinter, the metal nanoparticles grow, which in return causes a loss in the overall surface area from loss of the number of available reaction sites. At high temperatures, sintering as a thermally activated process occurs more rapidly and the energetics of the sample changes.
In automotive catalysis, for the catalytic converter, the exhaust gases from automobiles contain carbon monoxide, nitrogen oxide compounds, and other hydrogen compounds, which are detrimental to the atmosphere, and a catalyst is required to convert these gasses into less harmful gases, such as nitrogen, carbon dioxide, and water vapor. These reactions occur at very high temperatures, greater than 900 C, which causes the catalyst to degrade and not be as effective in converting these harmful gases to less harmful gases, requiring some state governments to mandate the testing of the exhaust gases from cars periodically to ensure the proper conversion is occurring.
Despite the importance of catalyst sintering, there is much yet to be understood about the mechanism of how catalysts sinter and to predict catalyst lifetimes under operating conditions. To better study how sintering occurs, a single crystal model catalyst will be employed for my experiments. A palladium catalyst, one of the three metals used in catalytic converters, was deposited on single crystal metal-oxide substrates. The three metal-oxides examined were Pd on alpha-alumina, quartz (silica), and yttria stabilized zirconia. Palladium nanoparticles with an average size of around 15 nm were created from a thin film of deposited pure palladium. After the creation of palladium nanoparticles, the samples were aged under two different conditions, under a vacuum atmosphere and a nitrogen atmosphere.
The vacuum atmosphere is used to measure the emission of adatoms from the nanoparticles. Any adatom emitted from the particle into the atmosphere will be removed from the system, causing a loss of mass to the system. The mass loss can be measured indirectly by using a scanning electron microscope and analyzing images of the system to calculate the mass lost from the system. From this mass loss, we can calculate the emission rate of Pd on the different supports to see how the support affects the emission of adatoms.
The other ageing condition is in nitrogen, where there would be no mass loss in the system, and from the scanning electron microscope we can analyze how rapidly particles grow. The faster the particles grow, the greater is the sintering rate, most likely from a greater rate of adatom emission. If the sintering rates correlate to emission rates under vacuum, then the emission step is critical to the understanding of catalyst sintering.
My research shows that the support does affect the emission and sintering rates. Results of evaporation at 900 C under vacuum showed the Pd on alpha-alumina evaporated the least, followed by Pd on YSZ then Pd on quartz. The results of sintering at 700 C showed that these trends also were true for sintering rates, with Pd on alumina sintering the least followed by Pd on YSZ and then Pd on quartz. These results show that the emission of adatoms can be measured by vacuum and the sintering rates do correlate.
With the emission rates correlating, there might be ways to learn how the support affects the amount of sintering. This might lead to the better design of catalysts which might reduce the amount of sintering.